The present invention pertains to a method that uses a polar liquid to charge nonconductive free-fibers to form an electrically-charged nonwoven fibrous web. The present invention also pertains to an apparatus that is suitable for making such a web.
Electrically-charged nonwoven webs are commonly used as filters in respirators to protect the wearer from inhaling airborne contaminants. U.S. Pat. Nos. 4,536,440, 4,807,619, 5,307,796, and 5,804,295 disclose examples of respirators that use these filters. The electric charge enhances the ability of the nonwoven web to capture particles that are suspended in a fluid. The nonwoven web captures the particles as the fluid passes through the web. The nonwoven web typically contains fibers that comprise dielectricxe2x80x94that is, nonconductivexe2x80x94polymers. Electrically-charged dielectric articles are often referred to as xe2x80x9celectretsxe2x80x9d, and a variety of techniques have been developed over the years for producing these products.
Early work relating to electrically-charging polymer foils is described by P. W. Chudleigh in Mechanism of Charge Transfer to a Polymer Surface by a Conducting Liquid Contact, 21 APPL. PHYS. LETT., 547-48 (Dec. 1, 1972), and in Charging of Polymer Foils Using Liquid Contacts, 47 J. APPL. PHYS., 4475-83 (October 1976). Chudleigh""s method involves charging a polyfluoroethylene polymer foil by applying a voltage to the foil. The voltage is applied through use of a conducting liquid that contacts the foil surface.
An early-known technique for making a polymeric electret in fibrous form is disclosed in U.S. Pat. No. 4,215,682 to Kubic and Davis. In this method, the fibers are bombarded with electrically-charged particles as they issue from a die orifice. The fibers are created using a xe2x80x9cmelt-blowingxe2x80x9d process, Where a stream of gas, which is blown at high velocity next to the die orifice, draws out the extruded polymeric material and cools it into a solidified fiber. The bombarded melt-blow fibers accumulate randomly on a collector to create the fibrous electret web. The patent mentions that filtering efficiency can be improved by a factor of two or more when the melt-blown fibers are electrically-charged in this fashion.
Fibrous electret webs also have been produced by charging them with a corona. U.S. Pat. No. 4,588,537 to Klaase et al., for example, shows a fibrous web that is continuously fed into a corona discharge device while positioned adjacent to one major surface of a substantially-closed dielectric foil. The corona is produced from a high-voltage source that is connected to oppositely-charged thin tungsten wires. Another high-voltage technique for imparting an electrostatic charge to a nonwoven web is described in U.S. Pat. No. 4,592,815 to Nakao. In this charging process, the web is brought into tight contact with a smooth-surfaced ground electrode.
Fibrous electret webs also may be produced from polymer films or foils, as described in U.S. Pat. Nos. Re. 30,782, Re. 31,285, and Re. 32,171 to van Turnhout. The polymer films or foils are electrostatically charged before being fibrillated into fibers that are subsequently collected and processed into a nonwoven fibrous filter.
Mechanical approaches also have been used to impart an electric charge to fibers. U.S. Pat. No. 4,798,850 to Brown describes a filter material that contains a mixture of two different crimped synthetic polymer fibers that have been carded into a fleece and then needled to form a felt. The patent describes mixing the fibers well so that they become electrically-charged during the carding. The process disclosed in Brown is commonly referred to as xe2x80x9ctribochargingxe2x80x9d.
Tribocharging also can occur when high-velocity uncharged jets of gases or liquids are passed over the surface of a dielectric film. In U.S. Pat. No. 5,280,406, Coufal et al. disclose that when jets of an uncharged fluid strike the surface of the dielectric film, the surface becomes charged.
A more recent development uses water to impart electric charge to a nonwoven fibrous web (see U.S. Pat. No. 5,496,507 to Angadjivand et al.). The electric charge is created by impinging pressurized jets of water or a stream of water droplets onto a nonwoven web that contains nonconductive microfibers. The resulting charge provides filtration-enhancing properties. Subjecting the web to an air corona discharge treatment before the hydrocharging operation can further enhance electret performance.
Adding certain additives to the web has improved the performance of electrets. An oily-mist resistant electret filter media, for example, has been provided by including a fluorochemical additive in melt-blown polypropylene microfibers; see U.S. Pat. Nos. 5,411,576 and 5,472,481 to Jones et al. The fluorochemical additive has a melting point of at least 25xc2x0 C. and a molecular weight of about 500 to 2500.
U.S. Pat. No. 5,908,598 to Rousseau et al. describes a method where an additive is blended with a thermoplastic resin to form a fibrous web. Jets of water or a stream of water droplets are impinged onto the web at a pressure sufficient to provide the web with filtration-enhancing electret charge. The web is subsequently dried. The additives may be (i) a thermally stable organic compound or oligomer, which compound or oligomer contains at least one perfluorinated moiety, (ii) a thermally stable organic triazine compound or oligomer which contains at least one nitrogen atom in addition to those in the triazine group, or (iii) a combination of (i) and (ii).
Other electrets that contain additives are described in U.S. Pat. No. 5,057,710 to Nishiura. The polypropylene electrets disclosed in Nishiura contain at least one stabilizer selected from hindered amines, nitrogen-containing hindered phenols, and metal-containing hindered phenols. The patent discloses that an electret that contains these additives can offer high heat-stability. The electret treatment was carried out by placing the nonwoven fabric sheet between a needle-like electrode and an earth electrode. U.S. Pat. Nos. 4,652,282 and 4,789,504 to Ohmori et al. describe incorporating a fatty acid metal salt in an insulating polymer to maintain high dust-removing performance over a long period of time. Japanese Patent Kokoku JP60-947 describes electrets that comprise poly 4-methyl-1-pentene and at least one compound selected from (a) a compound containing a phenol hydroxy group, (b) a higher aliphatic carboxylic acid and its metal salts, (c) a thiocarboxylate compound, (d) a phosphorous compound, and (e) an ester compound. The patent indicates that the electrets have long-term storage stability.
A recently-published U.S. patent discloses that filter webs can be produced without deliberately post-charging or electrizing the fibers or the fiber webs (see U.S. Pat. No. 5,780,153 to Chou et al.). The fibers are made from a copolymer that comprises: a copolymer of ethylene, 5 to 25 weight percent of (meth)acrylic acid, and optionally, though less preferably, up to 40 weight percent of an alkyl (meth)acrylate whose alkyl groups have from 1 to 8 carbon atoms. Five to 70% of the acid groups are neutralized with a metal ion, particularly zinc, sodium, lithium or magnesium ions, or mixtures of these. The copolymer has a melt index of 5 to 1000 grams (g) per 10 minutes. The remainder may be a polyolefin such as polypropylene or polyethylene. The fibers may be produced through a melt-blowing process and may be cooled quickly with water to prevent excess bonding. The patent discloses that the fibers have high static retention of any existing or deliberate, specifically induced, static charge.
The present invention provides a new method and apparatus, which are both suitable for making nonwoven fibrous electret webs. The method of making a nonwoven fibrous electret web comprises the steps: (a) forming one or more free-fibers from a nonconductive polymeric fiber-forming material; (b) spraying an effective amount of polar liquid onto the free-fibers; (c) collecting the free-fibers to form a nonwoven fibrous web; and (d) drying the fibers, the nonwoven web, or both, to form a nonwoven fibrous electret web.
The inventive apparatus includes a fiber-forming device that is capable of forming one or more free-fibers. A spraying system is positioned to allow a polar liquid to be sprayed onto the free-fibers. And a collector is positioned to collect the free-fibers in the form of a nonwoven fibrous web; while a drying mechanism is positioned to actively dry the resulting fibers or the nonwoven fibrous web.
The method of the present invention is different from known methods in that it involves spraying an effective amount of a polar liquid onto nonconductive free-fibers. After drying the nonwoven web, an electret charge becomes imparted on the fibers to create a nonwoven fibrous electret. There are a number of patents that disclose contacting a free-fiber with a liquid. In the known techniques, the free-fibers are exposed to the liquid for the purpose of quenching the fibers. The quenching step is employed for a variety of reasons, including to provide a noncrystalline mesomorphous polymer, to provide higher throughputs, to cool the fibers to prevent excess bonding, and to increase yarn uniformity (see U.S. Pat. Nos. 3,366,721, 3,959,421, 4,277,430, 4,931,230, 4,950,549, 5,078,925, 5,254,378, and 5,780,153). Although these patents generally disclose quenching the fiber with a liquid shortly after the fiber is formed, the patents do not indicate that an electret can be produced from spraying a polar liquid onto a nonconductive free-fiber. Applicants discovered that you need (i) a polar liquid, (ii) a nonconductive polymeric fiber-forming material, (iii) an effective amount of polar liquid, and (iv) a drying step to produce a nonwoven fibrous electret article.
The inventive method is advantageous in that the electret production steps are basically integral with the fiber-forming process and thus can conceivably reduce the number of steps for making a nonwoven fibrous electret web. Although subsequent charging techniques certainly may be employed in connection with the invention, an electret may be produced without the need or requirement for a charging operation that goes substantially beyond the web production process.
The apparatus of the invention differs from known fiber-producing apparatuses in that it includes a drying mechanism positioned to actively dry the fibers or the resulting nonwoven web. Known apparatuses have not employed a dryer because the quenching liquid apparently was used only in amounts sufficient to cool or quench the fibers and would passively dry by evaporation.
Finished articles produced in accordance with the method and apparatus of the invention may contain a persistent electric charge when dried, for example, on the collector. They do not necessarily need to be subjected to a subsequent corona or other charging operation to create the electret. The resulting electrically-charged nonwoven webs may be useful as filters and may maintain a substantially homogenous charge distribution throughout web use. The filters may be particularly suitable for use in respirators.
As used in this document:
xe2x80x9cfree-fiberxe2x80x9d means a fiber, or a polymeric fiber-forming material, in transit between a fiber-forming device and a collector.
xe2x80x9ceffective amountxe2x80x9d means the polar liquid is used in quantities sufficient to enable an electret to be produced from spraying the free-fibers with the polar liquid followed by drying.
xe2x80x9celectretxe2x80x9d means an article that possesses at least quasi-permanent electric charge.
xe2x80x9celectric chargexe2x80x9d means that there is charge separation.
xe2x80x9cfibrousxe2x80x9d means possessing fibers and possibly other ingredients.
xe2x80x9cnonwoven fibrous electret webxe2x80x9d means a nonwoven web that comprises fibers and that exhibits at least a quasi-permanent electric charge.
xe2x80x9cquasi-permanentxe2x80x9d means that the electric charge resides in the web under standard atmospheric conditions (22xc2x0 C., 101,300 Pascals atmospheric pressure, and 50% humidity) for a time period long enough to be significantly measurable.
xe2x80x9cliquidxe2x80x9d means the state of matter between a solid and a gas and includes a liquid in the form of a continuous mass, such as a stream, or in the form of a vapor or droplets such as a mist.
xe2x80x9cmicrofiberxe2x80x9d means fiber(s) that have an effective diameter of about 25 micrometers or less.
xe2x80x9cnonconductivexe2x80x9d means possessing a volume resistivity of about 1014 ohm.cm or greater at room temperature (22xc2x0 C.).
xe2x80x9cnonwovenxe2x80x9d means a structure, or portion of a structure, in which the fibers are held together by a means other than weaving.
xe2x80x9cpolar liquidxe2x80x9d means a liquid that has a dipole moment of at least about 0.5 Debye and a dielectric constant of at least about 10.
xe2x80x9cpolymerxe2x80x9d means an organic material that contains repeating linked molecular units or groups, regularly or irregularly arranged and includes homopolymers, copolymers, and blends of polymers.
xe2x80x9cpolymeric fiber-forming materialxe2x80x9d means a composition that contains a polymer, or that contains monomers that are capable of producing a polymer, and possibly other ingredients, and that is capable of being formed into solid fibers.
xe2x80x9csprayingxe2x80x9d means allowing the polar liquid to come into contact with the free-fiber by any suitable method or mechanism.
xe2x80x9cwebxe2x80x9d means a structure that is significantly larger in two dimensions than in a third and that is air permeable.